The Aux/IAA, Sl-IAA17 regulates quality parameters over tomato fruit development

Abstract

Auxin is known to be involved in all the stages of fruit development. Aux/IAAs are regulators of the auxin signaling at the transcription level. In a recent study, using RNAi strategy to limit the expression Sl-IAA17, it was shown that this tomato AuxIAA regulates fruit size mainly through altering the ploidy level of pericarp cells. Indeed, Sl-IAA17 down-regulated lines showed fruit with larger diameter, bigger volume and heavier weight than wild-type. The increase in fruit size was associated with thicker pericarp rather than larger locular spaces. The thicker pericarp was linked to larger cells harboring higher ploidy level, probably due to more active endoreduplication at the beginning of fruit development. The present report describes some additional phenotypes, not described in the initial article, among which are soluble solid content, juice pH, firmness, seed weight and fruit morphology.

The phytohormone auxin is known to regulate several aspects of plant development and Aux/IAA transcription factors play a pivotal role in auxin signaling. Auxin-dependent transcriptional regulation is mediated by the interplay between Auxin Response Factors (ARFs) and Aux/IAAs. In the tomato, a reference species for Solanaceae plants, 24 Aux/IAA and 22 ARF genes were identified.^1,2 Unlike the situation in the model plant Arabidopsis where the functional significance of Aux/IAAs was mainly gained by the analysis of gain-of-function mutants, uncovering the function in planta of tomato Aux/IAAs was mainly achieved through the characterization of downregulated lines.³ and refs herein; ^4-7 In particular, we have recently shown that down-regulation of Sl-IAA17 gene in tomato results in fruits with higher weight and thicker pericarp, probably as a result of enhanced cell expansion linked to an increased endoreduplication rate.⁷ We report here on additional phenotypes related to fruit ripening traits uncovered in these Sl-IAA17 down-regulated lines.

Fruit ripening is a complex developmental program characterized by striking metabolic and textural changes including changes in firmness, color and flavor, all known to be under the control of the plant hormone ethylene in the case of climacteric fruit like the tomato. Figure 1A shows that the soluble solid content (SSC) was higher in downregulated lines than in WT at the red stage (7 d after breaker), this difference being significant in the 2 lines displaying the strongest downregulation of Sl-IAA17.⁷ Recently, it has been observed that Sl-ARF4 under-expressing lines displaye higher soluble solids content at the ripening stages⁸ along with more starch accumulation and higher expression at the transcript level of the AGPase known to regulate the early step of starch synthesis.⁹ Interestingly, it has been observed that Sl-IAA17 protein is able to interact with several ARF proteins among which Sl-ARF4, Sl-ARF5, Sl-ARF6, Sl-ARF7 and Sl-ARF8.¹⁰ More detailed studies on the mechanistic of ARF and Aux/IAA association carried out with Arabidopsis proteins from the 2 showed recently that individual homo-oligomers of IAA17 and ARF5 exchange subunits to form hetero-oligomers that modulate transcription.¹¹ Whether the preferential interaction between protein partners might impact in a specific way the output of auxin signaling remains to be elucidated.

Figure 1.

Effects of SI-IAA17 RNAi down-expression (Rlines 1 to 3) on fruit quality parameters in comparison to wild types (WT). (A) soluble solids content; (B) pH value; (C) ethylene production during fruit during ripening: Breaker (Br), Br+1, Br+2, Br+3, Bn-4, Br+5, Br+6, Br+7 (figures = time in days); (D) fruit firmness; (E) fruit distal angle micro (as shown in the insert); (F) seed weight. Statistical analyses were performed using the t- test comparing WT with each Rline, *** P < 0.001, ** P < 0.01, * P < 0.1, n = 50 for A, D and E, n = 6 for B, C and F. Error bars are standard errors.

During fruit ripening the accumulation of sugars is correlated with a decrease in total acidity. Nevertheless the pH levels of the fruit juice were similar between WT and RNAi lines (Fig. 1B). Other markers of fruit ripening, such as the increase in ethylene production (Fig. 1C) and the loss of firmness (Fig. 1D) were not altered in the Sl-IAA17 RNAi lines compared to WT suggesting that the over-accumulation of sugars in the transgenic lines does not depend only on ripening acceleration.

In addition, the shape of RNAi fruits was altered compared to WT fruit, notably at the fruit bottom tip. Indeed, the measurement of the distal angle¹² revealed a difference between WT and transgenic fruit (Fig. 1E). This morphology parameter describes the angle of the fruit tip. The larger distal angle in Sl-IAA17 down-regulated lines may be connected to the larger fruit size of the RNAi lines,⁷ which look like well inflated balloons with a very round shape. This morphological trait could also be linked to the acceleration of the flower cap abscission (unpublished results). Indeed, if the flower cap stays longer on the young fruit it may constraint the fruit tissue spatial organization.

As tomato Sl-IAA9 downregulated lines displayed parthenocarpic fruits,³we analyzed the tomato seed number in the SlIAA17 RNAi lines. The seed number was unchanged in the RNAi lines compared to the WT, but a clear decrease of the RNAi seed weight was observed (Fig. 1F). Moreover, the germination rate of these seeds was slightly affected, i.e., only 80% Sl-IAA17 RNAi lines seeds were germinating compared to 100% WT (unpublished results). A more detailed analysis of the seed content will be developed to analyze the putative function of SlIAA17 during seed genesis.

These additional data shed light on the role that Sl-IAA17 in sugar metabolism in tomato fruit, the seed filling and the fruit shape. Further research is needed to better understand the regulations linking Sl-IAA17 to these phenotypes.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.